Conventional optical disk apparatuses reproduce information by irradiating a relatively weak light beam with a constant light amount onto an optical disk serving as an information carrier, and detecting the reflected light whose strength has been modulated by the optical disk. Moreover, information is recorded by irradiating a light beam whose light amount is modulated in accordance with the information to be recorded onto a recording material film on the optical disk (see for example JP S52-80802A).
In read-only optical disks, information is recorded in advance in a spiral shape with pits. Moreover, optical disks that can be recorded and reproduced are fabricated by forming a film made of an optically recordable/reproducible material (recording material film) by a method such as vapor deposition on a substrate surface having tracks of a spiral-shaped land-and-groove structure. In order to record information on the optical disk or to read information that has been recorded on the optical disk, a focus control is necessary that controls the focus of a light beam in a direction normal to the surface of the optical disk (referred to as “focus direction” in the following), such that the light beam is always in a predetermined converged state on the recording material film.
Referring to FIG. 14, the following is an explanation of the control operation of a conventional optical disk apparatus. As shown in FIG. 14, an optical head 110 includes a semiconductor laser 111, a coupling lens 112, a polarization beam splitter 113, a ¼ wavelength plate 114, a condensing lens 115 serving as a converging means, a focus actuator (referred to as “Fc actuator” in the following) 116 serving as a focus shifting means, a tracking actuator (referred to as “Tk actuator” in the following) 117 serving as a track shifting means, a detection lens 118, a cylindrical lens 119, and an optical detector 120.
The light beam that is emitted from the semiconductor laser 111 is converted into a parallel beam by the coupling lens 112. After this parallel beam has passed through the polarization beam splitter 113 and the ¼ wavelength plate 114, it is focused onto the information surface of the disk-shaped optical disk 101 by the condensing lens 115. Then, after the light beam reflected from the optical disk 101 has passed again through the condensing lens 115 and the ¼ wavelength plate 114, it is reflected by the polarization beam splitter 113. Then, after this reflected light beam has passed through the detection lens 118 and the cylindrical lens 119, it is irradiated onto the optical detector 120, which is partitioned into four sections. The condensing lens 115 is supported by an elastic member (not shown in the drawings), and is shifted by electromagnetic force in the focus direction, by letting an electrical current flow through the Fc actuator 116.
The optical detector 120 sends detected light amount signals to a focus error generator (referred to as “FE generator” in the following) 130 serving as a focus displacement signal detection means. The FE generator 130 uses the light amount signals from the optical detector 120 to calculate an error signal indicating the convergent state of the light beam on the information surface of the optical disk 101, that is, a focus error signal (referred to as “FE signal” in the following) corresponding to the positional displacement of the focus of the light beam with respect to the information surface of the optical disk 101. Then, the FE generator 130 sends this FE signal via a focus control filter (referred to as “Fc filter” in the following) 131, which performs a phase compensation, a driving selector 132, and a focus driver (referred to as “Fc driver” in the following) 137 to the Fc actuator 116, in order to stabilize the control operation of the focus control. The Fc actuator 116 drives the condensing lens 115 in the focus direction, such that the light beam converges in a predetermined state on the information surface of the optical disk 101.
A fixed driving signal generator 136 sends to the driving selector 132 a driving signal with which the Fc actuator 116 is mechanically put in its natural state, that is, a state in which no force is applied to the Fc actuator 116. If there is a positional displacement of the focus of the light beam with respect to the information surface of the optical disk 101, and this positional displacement needs to be corrected for the recording or reproducing of information, then the driving selector 132 sends the signal from the Fc filter 131 to the Fc driver 137. Based on the signal from the driving selector 132, the Fc driver 137 drives the Fc actuator 116. Then, the Fc actuator 116 drives the condensing lens 115 in a focus direction, such that the light beam converges on the information surface of the optical disk 101. In this state, it is said that “the focus control is in an operative state.” If it is not necessary to correct positional displacements of the focus of the light beam with respect to the information surface of the optical disk 101, then the driving selector 132 sends the signal from the fixed driving signal generator 136 to the Fc driver 137. In this state, it is said that “the focus control is in a inoperative state.” Based on the signal from the driving selector 132, the Fc driver 137 drives the Fc actuator 116. If the focus control is in the inoperative state, the Fc actuator 116 assumes its natural state.
The light amount signal from the optical detector 120 is also sent to the reflected light amount detector 161. Based on the light amount signal from the optical detector 120, the reflected light amount detector 161 detects a signal corresponding to the light amount reflected from the optical disk 101, and sends it to a focus anomaly detector (referred to as “Fc anomaly detector” in the following) 181. If the time for which the signal from the reflected light amount detector 161 is below a predetermined level continues for at least an anomaly detection time TW, then the internal status of the Fc anomaly detector 181 is set to a state indicating that the focus control has been lost. That is to say, the Fc anomaly detector 181 judges that a positional displacement of the focus of the light beam with respect to the information surface of the optical disk 101 has occurred.
As the density of optical disks increases, the optical disk and the condensing lens become closer when positioning the focus of the light beam on the information surface of the optical disk, and the risk of collisions between the optical disk and the condensing lens increases. The following two points are examples of the problems that may occur in this case:    (1) If the focus control is in the inoperative state, that is, if the optical disk apparatus is transported or moved, the condensing lens and the optical disk may collide easily.    (2) With the focus anomaly detection using the reflected light amount, the detection speed is slow, and the condensing lens and the optical disk may collide easily. In particular, if the optical head is moved to search the desired track with the focus of the light beam, or if external vibrations or shocks act on the optical disk apparatus, the focus control may be lost, and the condensing lens and the optical disk may collide.